Boiler structure

ABSTRACT

A boiler structure capable of efficiently alleviating or preventing corrosion and slagging on furnace walls in a furnace is provided. A circulating firing boiler structure is configured so that fuel and combustion air supplied into a furnace ( 11 ) from burners ( 12 ) disposed at a plurality of positions on furnace walls ( 11   a ) forming a rectangular cross section are combusted so as to form a swirling flow. Air-supplying parts ( 20 ) are disposed near flame-affected portions of furnace wall surfaces, where flames formed by the respective burners ( 12 ) approach or contact, to form regions having a higher air concentration than the peripheries thereof.

TECHNICAL FIELD

The present invention relates to a boiler structure compatible with coaland various fuels containing sulfur.

BACKGROUND ART

To reduce NO_(x) emissions, some recent boilers for use with fuels suchas coal and oil are supplied with air in multiple stages to form areducing-combustion zone where combustion proceeds in a reducingatmosphere between a main burner and an additional-air supplyingportion.

In the reducing-combustion zone, however, furnace wall surfaces areexposed to a severe corrosive environment where hydrogen sulfide, whichis a corrosive component, is produced in large amounts. Thisnecessitates maintenance such as spray coating onto furnace walls orregular replacement of furnace wall panels. Another concern is slagdeposition, since the reducing-combustion zone is a region with areducing atmosphere where the thermal load in the furnace is higher.

To cope with such problems, some known techniques are aimed atincreasing the oxygen concentration by supplying air toward the wallsurfaces of the furnace. According to one such technique, for example,burners are disposed at the four corners in a furnace having arectangular cross section to form a swirling flow, with each of theburners forming an air flow that is offset toward a furnace wall (forexample, see Patent Document 1).

According to a technique disclosed for a pulverized-coal-fired boilerhaving burners disposed in the centers of furnace walls to produce acirculating firing flame, nozzles are provided to supply a curtain ofair or a curtain of exhaust gas for deflecting the flames, therebypreventing slagging around the burners (for example, see Patent Document2).

Patent Document 1: the Publication of U.S. Pat. No. 6,237,513

Patent Document 2: Japanese Unexamined Patent Application, PublicationNo. HEI-7-119923

DISCLOSURE OF INVENTION

The conventional technique of Patent Document 1 above, however, cannoteffectively increase the oxygen concentration because oxygen containedin the air is consumed before it reaches a target wall surface. Inaddition, the flow rate at which the air is ejected must be increased toincrease the oxygen concentration. This is undesirable because it leadsto increased auxiliary power, including that of a compressor.

In the conventional technique of Patent Document 2, a curtain of air ora curtain of exhaust gas must be supplied at a flow rate high enough todeflect the flames. This is similarly undesirable because it leads toincreased auxiliary power, including that of a compressor.

Against such a backdrop, efficient alleviation or prevention ofcorrosion and slagging on furnace walls in a furnace is demanded of acirculating firing boiler structure that is compatible with coal andvarious fuels containing sulfur and that is configured so that fuel andcombustion air supplied into the furnace from burners disposed at aplurality of positions on furnace walls forming a rectangular crosssection are combusted so as to form a swirling flow.

An object of the present invention, which has been made in light of theabove circumstances, is to provide a boiler structure capable ofefficiently alleviating or preventing corrosion and slagging on furnacewalls in a furnace.

To solve the above problems, the present invention employs the followingsolutions.

A boiler structure according to the present invention is a circulatingfiring boiler structure configured so that fuel and combustion airsupplied into a furnace from burners disposed at a plurality ofpositions on furnace walls forming a rectangular cross section arecombusted so as to form a swirling flow. Air-supplying parts aredisposed near flame-affected portions of furnace wall surfaces, whereflames formed by the respective burners approach or contact, to formregions having a higher air concentration than the peripheries thereof.

With this boiler structure, in which the air-supplying parts aredisposed near the flame-affected portions of the furnace wall surfaces,where the flames formed by the respective burners approach or contact,to form the regions having a higher air concentration than theperipheries thereof, the regions having a higher air concentration canbe formed by supplying low-flow-rate air, which requires low auxiliarypower, to regions where there is concern over corrosion or slagging onthe furnace wall surfaces.

In the above invention, the regions having a higher air concentrationare preferably formed so as to cover a reducing-combustion zone insidethe furnace in a vertical direction. This allows the regions having ahigher air concentration to be formed by supplying air at a low flowrate in upper and lower regions where there is concern over corrosion orslagging in the furnace.

In the above invention, the air-supplying parts preferably introducelow-pressure secondary burner air from the adjacent burners throughbypass routes. This avoids a significant change in structure or anincrease in the number of components, thus simplifying the structure.

In the above invention, the air-supplying parts are preferably disposedaround deslagger nozzles. The air-supplying parts can then form theregions having a higher air concentration on the furnace wall surfacesin regions where slagging tends to occur and can also cool theperipheries of deslagger-nozzle insertion units, which are exposed tosevere thermal conditions.

According to the invention described above, in the circulating firingboiler structure configured so that fuel and combustion air arecombusted so as to form a swirling flow, the air-supplying parts supplyair at a low flow rate to the vicinities of the flame-affected portionsof the furnace walls, where there is concern over corrosion or slagging,in the furnace to form the regions having a higher air concentrationthan the peripheries thereof. This boiler structure can thereforemaintain a high oxygen concentration on and around the flame-affectedportions without the need for a high auxiliary power for increasing theflow rate of the supplied air.

Accordingly, an air layer having a higher oxygen concentration is formedon and around the flame-affected portions in the furnace, so that thereducing atmosphere is partially replaced by an oxidizing atmosphere. Asa result, corrosion and slagging can efficiently be alleviated orprevented. The above invention is particularly effective in alleviatingslagging of coal-fired boilers and is particularly effective inimproving corrosion resistance against hydrogen sulfide of boilerscompatible with various fuels containing sulfur.

In addition, if the air used by the air-supplying parts is low-pressuresecondary burner air introduced from the adjacent burners through bypassroutes, a significant change in boiler structure or an increase in thenumber of components can be minimized, thus simplifying the structure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a horizontal sectional view of an embodiment of a boilerstructure according to the present invention, showing areducing-combustion zone in a furnace.

FIG. 1B is a perspective view of the embodiment of the boiler structureaccording the present invention, showing its schematic outline.

FIG. 2A is a sectional view of the furnace, showing an exemplarystructure of an air-supplying part disposed on a deslagger-nozzleinsertion unit.

FIG. 2B is a diagram as viewed from arrow A of FIG. 2A, showing theexemplary structure of the air-supplying part disposed on thedeslagger-nozzle insertion unit.

FIG. 3A is a horizontal sectional view of a first modification of theboiler structure according to the present invention, showing areducing-combustion zone in a furnace.

FIG. 3B is a perspective view of the first modification of the boilerstructure according to the present invention, showing its schematicoutline.

FIG. 4A is a horizontal sectional view of a second modification of theboiler structure according to the present invention, showing areducing-combustion zone in a furnace.

FIG. 4B is a perspective view of the second modification of the boilerstructure according to the present invention, showing its schematicoutline.

FIG. 5 is a schematic longitudinal sectional view of a boiler structurethat combusts fuel with combustion air supplied in multiple stages.

EXPLANATION OF REFERENCE SIGNS

-   10: boiler-   11: furnace-   11 a: furnace wall-   12: burner-   20: air-supplying part (air-supplying nozzle)-   30: deslagger-nozzle insertion unit

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of a boiler structure according to the present inventionwill now be described with reference to the drawings.

Referring to FIG. 5, a boiler 10 combusts fuel by supplying combustionair into a furnace 11 in multiple stages to reduce NO_(x) emissions. Inthe multistage supply of this case, the combustion air is supplied intothe furnace 11 in two stages, that is, from burner portions Ba that areregions where a plurality of burners 12 are disposed and additional-airsupplying portions Aa that are regions where additional-air supplyingnozzles 13 are disposed above the burner portions Ba. In the boiler 10,specifically, as a measure against NO_(x) emissions, the two-stagecombustion is performed in a reducing-combustion zone and acomplete-combustion zone by initially supplying about 70% of therequired amount of combustion air from the burner portions Ba beforesupplying the rest, namely, about 30%, from the additional-air supplyingportions Aa.

Referring to FIG. 1A, for example, the boiler 10 described above is aswirling-combustion boiler in which the furnace 11 has a rectangularcross section. The swirling-combustion boiler 10 is configured so thatfuel and combustion air supplied from the plurality of burners 12, whichare disposed on furnace walls 11 a, into the furnace 11 are combusted soas to form a swirling flame in the furnace 11.

In the exemplary structure of the 8-cornered furnace shown in FIG. 1A,the burners 12, which are disposed at eight positions in a horizontalcross section, supply fuel and combustion air so as to form two adjacentswirling flows in the furnace 11.

In this embodiment, the boiler 10 includes air-supplying parts 20disposed near flame-affected portions of the furnace wall surfaces(furnace walls 11 a), where flames formed by the respective burners 12approach or contact, to form regions having a higher air concentrationthan the peripheries thereof. Specifically, in the horizontal crosssection of the 8-cornered furnace shown in FIG. 1A, one air-supplyingpart 20 is provided at an appropriate position on each of the furnacewalls 11 a, which form, for example, a rectangle; that is, a total offour air-supplying parts 20 are provided.

The formation of the regions having a higher air concentration meansformation of regions having a higher oxygen concentration. In theseregions, therefore, the reducing atmosphere is replaced by an oxidizingatmosphere.

That is, the air-supplying parts 20 are provided on the furnace walls 11a in the furnace 11 to supply air at a low flow rate from sites wherethere is concern over corrosion or slagging, thus forming the regionshaving a higher air concentration than the peripheries thereofsubstantially along the wall surfaces. In other words, the regionshaving a higher air concentration than the peripheries thereof areformed not by supplying air toward the furnace walls 11 a in the regionswhere there is concern over corrosion or slagging at a relatively highflow rate (for example, 40 m/sec or more), but by supplying air from theair-supplying parts 20 provided on the furnace walls 11 a in the regionswhere there is concern over corrosion or slagging at a low flow rate(for example, about 10 m/sec).

For example, the air-supplying parts 20 are nozzles for forming theregions having a higher air concentration by supplying low-pressuresecondary burner air introduced from the adjacent burners 12 throughbypass routes into the furnace 11 at a low flow rate. In a plan view ofthe furnace 11, the air supplied from the air-supplying parts 20 formsthe regions having a higher air concentration along the furnace walls 11a near the flame-affected portions. In addition, the air-supplying parts20 are provided in a plurality of stages in the vertical direction ofthe furnace 11 to cover the reducing-combustion zone inside the furnacein the vertical direction.

In the reducing-combustion zone, not only are the wall surfaces 11 aexposed to a severe corrosive environment, but also there is concernover slag deposition, because this zone is a region where hydrogensulfide, which is a corrosive component, is produced in large amountsand is also a reducing region where the thermal load in the furnace 11is higher. In the reducing-combustion zone, therefore, the air-supplyingparts 20 are provided in the peripheries of the portions on the furnacewalls 11 a where the flames approach or contact, at substantially thesame heights as the burners 12. This is because the flame-affectedportions of the furnace walls 11 a are formed at substantially the sameheights as the burners 12 since the flames are formed so as to extendfrom the burners 12 substantially in the horizontal direction.

In addition, the flame-affected portions of the furnace walls 11 a areformed at a plurality of positions in the vertical direction because theburners 12 in the reducing-combustion zone are usually provided in aplurality of stages in the vertical direction. Accordingly, theair-supplying parts 20 are provided in the vertical direction in thenumber of stages that is equal to the number of stages of the burners12, in other words, the number of stages of the flames formed in thevertical direction. This allows the regions having a higher airconcentration to be formed by supplying air at a low flow rate in upperand lower regions where there is concern over corrosion or slagging inthe furnace 11.

In the reducing-combustion zone, as a result, the air supplied at a lowflow rate from the air-supplying parts 20 provided near theflame-affected portions, which are formed by the burners 12, of thefurnace walls 11 a forms the regions having a higher air concentrationthan the peripheries thereof, so that the air functions as an air layerin the peripheries of the flame-affected portions to insulate thefurnace walls 11 a from the flames. This reduces the thermal effect andso on of the flames and also makes the atmosphere partially oxidizing,thus alleviating or preventing corrosion and slagging on the furnacewalls 11 a in the regions where the flame-affected portions wouldotherwise be formed.

In addition, low-flow-rate air, which requires low auxiliary power, canbe used because the air-supplying parts 20 supply the air from thevicinities of the flame-affected portions to the peripheries thereof.That is, high-pressure, high-flow-rate air does not have to be suppliedusing, for example, a compressor that operates with high power, unlikethe case where the air is supplied toward a remote position. Inparticular, the use of low-pressure secondary air introduced from theburners 12 reduces the auxiliary power and also avoids a significantchange in structure or an increase in the number of components, thussimplifying the structure.

Referring to FIG. 1B, for example, the air-supplying parts 20 areprovided around deslagger nozzles 31 in deslagger-nozzle insertion units30 between the burner portions Ba and the additional-air supplyingportions Aa. The deslagger-nozzle insertion units 30 are devices forremoving slag deposited on the furnace walls 11 a. Referring to FIG. 2A,for example, the deslagger-nozzle insertion units 30 clean the furnacewalls 11 a with steam ejected from the deslagger nozzles 31, which areinserted in the furnace 11.

That is, it is effective to form the regions having a higher airconcentration by supplying air because the deslagger-nozzle insertionunits 30 are provided at sites where there is concern over slagdeposition because of the high thermal load due to the reducingatmosphere in the furnace 11.

An exemplary structure of the air-supplying parts 20 provided around thedeslagger-nozzle insertion units 30 will now be described with referenceto FIGS. 2A and 2B.

In FIG. 2A, the deslagger nozzle 31 is attached to the deslagger-nozzleinsertion unit 30 by inserting the deslagger nozzle 31 in a nozzle hole32 extending through the furnace wall 11 a. The deslagger nozzle 31 issupplied with steam to be ejected for removing slag through a steam duct33. Reference numeral 34 in the drawing denotes a seal member providedbetween a nozzle body 21 of the air-supplying nozzle (air-supplyingpart) 20, to be described below, and the deslagger nozzle 31.

The air-supplying nozzle 20, on the other hand, has an air flow channel22 formed of an annular space between the deslagger nozzle 31 and thenozzle hole 32, and the nozzle body 21 has a circular flange 21 a at oneend of its cylindrical shape and is attached to the furnace 11. Thenozzle body 21 is fixed to, for example, the circumferential surface ofthe deslagger nozzle 31 with the seal member 34 disposed therebetween,and the flange 21 a in the furnace 11 faces the furnace wall 11 a so asto be substantially parallel thereto with a predetermined distancetherebetween. Hence, air supplied from the nozzle body 21 into thefurnace 11 collides with the flange 21 a, thus flowing outward along thefurnace wall 11 a around the entire circumference in the circumferentialdirection.

The air-supplying nozzle 20 has a wind box 23 provided outside thefurnace 11. The wind box 23 communicates with the nozzle body 21 in thefurnace 11 through the air flow channel 22 to supply air from an airsupply 24. In this case, the air supply 24 used is preferably, forexample, the low-pressure secondary air introduced from the burners 12,although the primary air or compressed air may be used if necessary.

The air-supplying nozzle 20 can form a region having a higher airconcentration along the furnace wall 11 a of the furnace 11 in a regionwhere slagging tends to occur and can also cool the periphery of thedeslagger-nozzle insertion unit 30, which is exposed to severe thermalconditions. Accordingly, an air layer having a higher air concentrationthan the periphery thereof is formed around the furnace wall 11 a in aregion where slagging tends to occur, so that a partial oxidizingatmosphere can prevent or alleviate corrosion of the wall surface, thusextending the life of the furnace wall.

In addition, the air supplied into the nozzle body 21 of theair-supplying part 20 flows beside the circumferential surface of thedeslagger nozzle 31. The air flow can therefore cool, for example, theseal member 34, which is exposed to severe thermal conditions.

Furthermore, as the air concentration is increased in the vicinity ofthe furnace wall 11 a, on which the air-supplying nozzle 20 is provided,the oxygen concentration is increased, thus creating an oxidizingatmosphere. The oxidizing atmosphere can alleviate slagging because themelting temperature of slag is increased thereby.

In this boiler structure, the air-supplying parts 20 are disposed nearthe flame-affected portions of the furnace walls 11 a, where the flamesformed by the respective burners 12 approach or contact, to form theregions having a higher air concentration than the peripheries thereof.Because the oxygen concentration is increased around the flame-affectedportions, the reducing atmosphere is partially replaced by an oxidizingatmosphere. As a result, corrosion and slagging can be alleviated orprevented, thus extending the life of the wall surfaces. This boilerstructure is particularly effective in alleviating slagging ofcoal-fired boilers and is particularly effective in improving corrosionresistance of boilers compatible with various fuels containing sulfur.

The optimum positions of the air-supplying parts 20 in the horizontalcross section vary depending on the conditions, including the shape ofthe furnace 11, the positions and number of the burners 12, and the typeof swirling flame formed. That is, the regions of the flame-affectedportions of the furnace walls 11 a, where the flames formed by therespective burners 12 approach or contact, vary with, for example, thearrangement of the burners 12 and the type of swirling flame formed.Accordingly, the positional relationship between the burners 12 and theair-supplying parts 20 differs between different boiler structures, forexample, the 8-cornered furnace shown in FIGS. 1A and 1B and 4-corneredfurnaces shown in FIGS. 3A and 3B and FIGS. 4A and 4B.

In the exemplary structure shown in FIGS. 1A and 1B, the furnace 11 isrectangular, and four burners 12 are disposed on each of the twoopposing long sides to form two swirling flows on the left and right. Inthis case, the burners 12 are tilted toward substantially the centers ofthe respective swirling flows, that is, toward substantially the centersof squares formed by dividing the rectangle in half, so that the twoswirling flows each have a substantially oval shape.

In this case, therefore, the flame-affected portions, where the flamesapproach or contact, are formed near two corners and the centers of thelong sides, and the air-supplying parts 20 are provided at fourpositions so as to cover these regions.

In an exemplary structure (first modification) shown in FIGS. 3A and 3B,the furnace 11 is square, and the burners 12 are disposed at fourpositions offset from the centers of the respective sides to form asingle swirling flow. In this case, the swirling flow is formed by theoffset of the burners 12 because the burners 12 are directed toward theopposite wall surfaces. In this arrangement of the burners 12, theflames flow toward the vicinities of the centers of the wall surfaces onthe downstream side of the swirling flow under the effect of the flamesformed on the upstream side.

In this case, therefore, the flame-affected portions are near thecenters of the respective sides, and accordingly the air-supplying parts20 are provided at four positions in the centers of the respective sidesso as to cover these regions.

In an exemplary structure (second modification) shown in FIGS. 4A and4B, the furnace 11 is square, and the burners 12 are disposed at thefour corners to form a single swirling flow. In this case, theflame-affected portions are near the centers of the respective sides,and accordingly the air-supplying parts 20 are provided at fourpositions in the centers of the respective sides so as to cover theseregions.

Thus, the optimum positions of the air-supplying parts 20 may beselected on the basis of, for example, the arrangement of the burners12.

The present invention is not limited to the embodiments described above;modifications are permitted so long as they do not depart from thespirit of the invention.

1: A circulating firing boiler structure configured so that fuel andcombustion air supplied into a furnace from burners disposed at aplurality of positions on furnace walls forming a rectangular crosssection are combusted so as to form a swirling flow, whereinair-supplying parts are disposed near flame-affected portions of furnacewall surfaces, where flames formed by the respective burners approach orcontact, to form regions having a higher air concentration than theperipheries thereof. 2: The boiler structure according to claim 1,wherein the regions having a higher air concentration are formed so asto cover a reducing-combustion zone inside the furnace in a verticaldirection. 3: The boiler structure according to claim 1, wherein theair-supplying parts introduce low-pressure secondary burner air from theadjacent burners through bypass routes. 4: The boiler structureaccording to claim 1, wherein the air-supplying parts are disposedaround deslagger nozzles. 5: The boiler structure according to claim 2,wherein the air-supplying parts introduce low-pressure secondary burnerair from the adjacent burners through bypass routes. 6: The boilerstructure according to claim 2, wherein the air-supplying parts aredisposed around deslagger nozzles. 7: The boiler structure according toclaim 3, wherein the air-supplying parts are disposed around deslaggernozzles.